The invention relates to the field of acoustic attenuation, and specifically to an apparatus and method for the active cancellation of acoustic waves radiating from a fluid-containing enclosure. The present invention is particularly useful for reducing noise generated by industrial equipment having a noise source connected to a radiating surface via a fluid transmission path, such as an oil-filled transformer of a type commonly used in residential power sub-stations.
Many current industrial systems, including transformers and refrigeration units, generate noise at levels that are unacceptable for residential areas, often exceeding limits set by local ordinances. In an oil-filled transformer, for example, this noise is the result of vibrations induced by the core of the transformer passing through the surrounding oil and radiating out to the atmosphere through the walls of the oil-containing tank. It is often economically necessary to locate such a transformer close to a residential area; however, the constant hum produced by the transformer is a source of irritation to those who live within range of the noise. Accordingly, there is a need for an economical means for reducing the noise generated by such equipment.
Early approaches to the problem of excessive noise generated by fluid-filled transformers included installation of quieter transformers or de-rating the load carried by existing transformers. The former approach involves a significant capital expenditure, usually including price premiums for quieter designs; while the latter approach is not practical for many existing installations. Yet another approach involves building sound walls around transformers, but such physical barriers are expensive to construct and generally interfere with the high voltage lines coming into the sub-station.
More recent approaches to the problem focused on noise cancellation systems. One such system involves the use of loudspeakers to produce an external control volume in an effort to cancel the airborne noise radiation around the industrial system. Such loudspeaker-based systems have proven to be unreliable and often experience an unsatisfactory degree of variability as a result of changes in weather and ambient noise. A system of this type is described in Samuel E. Craig and O. L. Angevine, Active Control of Hum from Large Power Transformers--The Real World, Second Conference on Recent Advances in Active Control of Sound and Vibration 279-290 (1993).
A more satisfactory approach to reducing excessive noise radiated from a fluid-bearing enclosure involves active attenuation of fluid-borne noise through the use of a canceling vibration applied to the fluid. One such method is described in PCT patent application WO 81/01479, filed by George B. Chaplin on Nov. 10, 1980. The Chaplin application discloses the use of a harmonic generator and a vibrator device to introduce a secondary vibration into a fluid-containing transformer tank, thereby reducing the hum produced by a primary vibration emanating from the transformer core. The harmonic generator taps an AC signal from a power line feeding the transformer core and uses a pass filter, a variable phase shifter and a variable amplitude control to produce signals corresponding to the 2f, 4f and 6f harmonics of the AC signal. These signals are then fed to a summation means and a power amplifier prior to being input to the vibrator device. As the vibrator device is activated, a series of microphones or vibration sensors are monitored to ascertain the effect on the hum produced by the transformer. Incremental adjustments may then be made to the individual harmonics to attain an optimum noise level and compensate for changes occurring in the primary vibration generated by the transformer core.
While representing a promising approach, the Chaplin invention requires an excessively complex attenuation system to achieve an acceptable level of cancellation performance. Acceptable cancellation performance is generally defined as reducing by at least one-half the sound power radiated by an enclosure. Chaplin is deficient in that the disclosed invention processes the entire set of pressure fluctuation patterns in an enclosed liquid, as opposed to extracting only those components that are responsible for the radiated sound. This results in a significant increase in system complexity, since the actuators must try to match (with a 180.degree. phase shift) very complex spatial patterns detected by the vibration sensors.